When was the first dark matter evidence reported in a galaxy cluster?

In 1933, astronomer Fritz Zwicky analyzed the Coma Cluster and found that the visible galaxies did not contain enough mass to hold the cluster together. He inferred the presence of unseen “dunkle Materie,” or dark matter. This was one of the earliest major clues that much of the universe’s mass is invisible. Dark matter remains one of cosmology’s biggest unsolved problems.